Big problem in my area, you see all these 'photographers' aka GWCs (guy(gal)s with cameras) pop up after they have a shoot with the original photographer whos been around for years, and they start shooting clients in the same nature at the same locations. It can be frustrating and you will think they are 'stealing' clients but really would you have gotten that client yourself? A portriat shoot isn't something people usually look for its more of an impulse buy unless you are trying to become a model hardcore.
The best thing to do is push yourself out further and advertise more, have a better website, better business card, etc. they don't know how many clients you've had until they look into you and contact you. And the average client doesn't understand the artistic side of things, you may have better photographs according to the art critics of the world and other photographers but most clients I meet cannot tell the difference unless they are right next to each other.